1. Field of the Invention
The present invention relates to an apparatus and method for detecting very small distances, and more particularly to proximity sensing with gas flow.
2. Related Art
Many automated manufacturing processes require the sensing of the distance between a manufacturing tool and the product or material surface being worked, often referred to as a “work piece” (e.g., a semiconductor wafer, a flat panel display substrate, or the like). In some situations, such as lithography (e.g., maskless lithography, immersion lithography, photolithography, etc), the distance must be measured with accuracy approaching a nanometer.
The challenges associated with creating a proximity sensor of such accuracy are significant, particularly in the context of lithography systems. In the lithography context, in addition to being non-intrusive and having the ability to precisely detect very small distances, the proximity sensor can not introduce contaminants or come in contact with the work piece, typically the semiconductor wafer, flat panel display, or the like. Occurrence of either situation may significantly degrade or ruin the work piece.
Different types of proximity sensors are available to measure very small distances. Examples of proximity sensors include capacitance sensors and optical sensors. These proximity sensors have serious shortcomings when used in the lithography systems because physical properties of materials deposited on wafers or substrates may impact the precision of these devices. For example, capacitance gauges, being dependent on the concentration of electric charges, can yield spurious proximity readings in locations where one type of material (e.g., metal) is concentrated. Another class of problems occurs when exotic wafers made of non-conductive and/or photosensitive materials, such as Gallium Arsenide (GaAs) and Indium Phosphide (InP), are used. Further problems can result from light interacting with under-the-surface parts of the wafers or substrates, which can cause spurious reflections and unwanted interference patterns. In these cases, capacitance and optical sensors are not optimal.
An alternative approach to proximity sensing uses a gas gauge sensor. A gas gauge sensor is not vulnerable to concentrations of electric charges or electrical, optical, and other physical properties of a substrate surface. Current semiconductor manufacturing requires that proximity be gauged with high precision on the order of nanometers. Gas gauge technology can be an accurate method of measuring the distance to a surface in a close proximity. Gas gauges are insensitive to the optical or electrical properties of the material being measured. Distance accuracy can be on the order of nanometers. Gas gauges can be employed in the lithography systems to establish a distance to a top surface of the wafer or substrate.
Focus precision requirements have tightened dramatically as the printed feature size shrinks. One problem with gas gauge proximity sensors is that they can be sensitive to low frequency external acoustical interference and sensor offset errors. Lithography tool exposure system stages often employ interferometers to control position, and these can be sensitive to gasses of different content, pressure and temperature.
Therefore, a gas gauge proximity sensor is desired that is substantially insensitive to external noise.